remove elliptic.P224 usage

Fedora/RedHat distros comply with US patent law and remove this curve,
which makes it impossible to run ethereum with distro provided Golang.

File crypto/ecies/README claims it is unsupported anyway.

crypto/ecies/ecies_test.go

@@ -407,11 +407,6 @@ type testCase struct {
 }
 
 var testCases = []testCase{
-	testCase{
-		Curve:    elliptic.P224(),
-		Name:     "P224",
-		Expected: false,
-	},
 	testCase{
 		Curve:    elliptic.P256(),
 		Name:     "P256",